What is geographic data?

• Thus far: we’ve been working with data (largely) in tabular data frames, where each column represents data “attributes”
• Geographic data: includes information on location - specifically, where the observation is located on the Earth’s surface

Maps!

Longitude and latitude: the basics

Geographic coordinates

• Longitude (X) and latitude (Y) coordinates can be expressed as:
• Degrees minutes seconds (e.g. \(97^{\circ}21'37''W\), \(32^{\circ}42'38''N\))
• Decimal degrees (e.g. \(-97.36\), \(32.71\))

Conversion between DMS and decimal degrees:

\[DD = D + \frac{M}{60} + \frac{S}{3600}\]

Coordinate systems

• Geographic coordinate system: coordinate system based on latitude and longitude (coordinates on a sphere)
• Projected coordinate system: coordinate system projected onto a two-dimensional surface (coordinates on a plane)

Spheres and planes

Map projections

Tiled mapping and Web Mercator

Map projections and distortion

Map projections and distortion

Vector data

Raster data

Vector data: CSV files

• Example: Starbucks locations in Chicago
• Data download: http://personal.tcu.edu/kylewalker/data/chicago_starbucks.csv

Let’s make a map!

• CartoDB: https://cartodb.com/signup

Mapping point data

Vector data: the shapefile

• Shapefile: common format for encoding vector geographic data
• Three required files: .shp, .dbf, and .shx; .prj recommended

Vector data: GeoJSON

• Mexico’s states as GeoJSON

Mapping polygons

Should you map?

• Beware the “amazing map”…

Maps vs. charts

• For discussion: which visualization is more effective for showing differences in our data?

Chart for comparison